Isolating a thick disc in Andromeda

A team of astronomers from the UK, the US and Europe have identified a
thick stellar disc in the nearby Andromeda galaxy for the first time.
The discovery and properties of the thick disc will constrain the
dominant physical processes involved in the formation and evolution of
large spiral galaxies like our own Milky Way.

Optical image of The Andromeda galaxy (M31) (credit Robert Gendler)

By analysing precise measurements of the velocities of individual bright
stars within the Andromeda galaxy using the Keck telescope in Hawaii,
the team have managed to separate out stars tracing out a thick disc
from those comprising the thin disc, and assess how they differ in
height, width and chemistry.

Spiral structure dominates the morphology of large galaxies at the
present time, with roughly 70% of all stars contained in a flat stellar
disc. The disc structure contains the spiral arms traced by regions of
active star formation, and surrounds a central bulge of old stars at the
core of the galaxy. ``From observations of our own Milky Way and other
nearby spirals, we know that these galaxies typically possess two
stellar discs, both a `thin' and a `thick' disc,'' explains the leader
of the study, Michelle Collins, a PhD student at Cambridge's Institute
of Astronomy. The thick disc consists of older stars whose orbits take
them along a path that extends both above and below the more regular
thin disc. ``The classical thin stellar discs that we typically see in
Hubble imaging result from the accretion of gas towards the end of a
galaxy's formation'', Collins continues, ``whereas thick discs are
produced in a much earlier phase of the galaxy's life, making them ideal
tracers of the processes involved in galactic evolution.''

Schematic representation of a thick disc structure. The thick disc is formed of stars that are typically much older than those in the thin disc, making it an ideal probe of galactic evolution (Credit: Amanda Smith, IoA graphics officer)

Currently, the formation process of the thick disc is not well
understood. Previously, the best hope for comprehending this structure
was by studying the thick disc of our own Galaxy, but much of this is
obscured from our view. The discovery of a similar thick disk in
Andromeda presents a much cleaner view of spiral structure. Andromeda is
our nearest large spiral neighbour -- close enough to be visible to the
unaided eye -- and can be seen in its entirety from the Milky Way.
Astronomers will be able to determine the properties of the disk across
the full extent of the galaxy and look for signatures of the events
connected to its formation. It requires a huge amount of energy to stir
up a galaxy's stars to form a thick disc component, and theoretical
models proposed include accretion of smaller satellite galaxies, or more
subtle and continuous heating of stars within the galaxy by spiral arms.

``Our initial study of this component already suggests that it is
likely older than the thin disc, with a different chemical
composition'' commented UCLA Astronomer, Mike Rich,
``Future more detailed observations should enable us to unravel the
formation of the disc system in Andromeda, with the potential to apply
this understanding to the formation of spiral galaxies throughout the
Universe.''

Ages and orientations of the stellar components of disc galaxies. The halo (or spheroid) contains the oldest populations, followed by the thick stellar disc. The thin disc typically contains the youngest generations of stars. (Credit: RAVE collaboration)

``This result is one of the most exciting to emerge from the larger
parent survey of the motions and chemistry of stars in the outskirts of
Andromeda,'' said fellow team member, Dr. Scott Chapman, also at the
Institute of Astronomy. ``Finding this thick disc has afforded us a
unique and spectacular view of the formation of the Andromeda system,
and will undoubtedly assist in our understanding of this complex process.''

This study was published in Monthly Notices of the Royal Astronomical
Society (see the accepted paper) by Michelle Collins, Scott
Chapman and Mike Irwin from the Institute of Astronomy, together with
Rodrigo Ibata from L'Observatoire de Strasbourg, Mike Rich from
University of California, Los Angeles, Annette Ferguson from the
Institute for Astronomy in Edinburgh, Geraint Lewis from the University
of Sydney, and Nial Tanvir and Andreas Koch from the University of
Leicester.